Population genetic SNP analysis of marble and brown trout in a hybridization zone of the Adriatic watershed in Slovenia

Abstract

Marble trout (Salmo marmoratus) is an extensively managed salmonid taxon threatened by historical introductions of non-native brown trout (Salmo trutta) of different origins into habitats in the northern part of its distribution, namely the Po river system and the Slovenian part of the Adriatic watershed. Hybridization between marble trout and brown trout results in fertile offspring, creating an extensive zone of genetic mixing, with varying amount of introgression. A rehabilitation programme for marble trout has been undertaken in Slovenia for two decades, dependent upon eight non-introgressed marble trout populations inhabiting isolated streams in the upper Soča river valley. MtDNA and a new marker system based on nuclear DNA, designed for identification of trouts and their hybrids in the genus Salmo, were used to determine the extent of introgressive hybridization between marble and brown trout across a large area of the Adriatic watershed in Slovenia. Individual admixture coefficients, the posterior probability of each individual belonging to one or other of the parental species and user-defined categories of admixture were determined along with basic population genetic parameters, revealing variable intensity and patterns of individual introgression throughout the zone of hybridization. In most of the populations analysed, hybridization of native marble trout with the introduced Atlantic lineage of brown trout was observed. In the upper Soča river system a high proportion of genetically non-introgressed (pure) marble trout was found, enabling selection of these fish for supplementary material in the reintroduction programme. Conclusions regarding management of marble trout in Slovenia can be inferred from the patterns observed in the present study: (1) transfer of pure populations to fishless or fished-out streams is recommended, (2) to sustain genetic diversity of the species, non-introgressed individuals sorted out from the zone of hybridization should be used to produce juveniles for supplementation stocking in fish farms, and (3) extension of the marble trout repopulation programme to highly introgressed populations in small unmanaged streams, which represent a potential source for downstream genetic ‘contamination’.